The field of the invention relates generally to imaging devices. More particularly, the present disclosure is directed to complementary metal-oxide-semiconductor (“CMOS”)-based imaging sensors capable of a wide range of operation.
Imagers typically include an array of pixels that contain light-sensitive elements commonly known as photodetectors, such as CMOS or charge-coupled device (“CCD”) sensors. In general, photodetectors accumulate charge in accordance with the incident light during what is known as an integration period. If a photodetector becomes full, or saturated, before the end of the integration period, additional light from a bright scene striking the photodetector does not accumulate any further charge, and stores no further information. As such, many imaging applications require sensors with a wide dynamic range in order to capture inter- and intra-scene lighting variation with high resolution and sensitivity. Specifically, dynamic range refers to the ratio of the maximum signal output to the smallest signal output of a given sensor, which is determined by noise.
CMOS image sensors first came to the fore in relatively low-performance applications where scene dynamic range was low, and moderate to high noise levels could be tolerated. A CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, star trackers, motion detection systems, image stabilization systems, and high-definition television imaging devices.
The advantages of CMOS imagers over CCD imagers are that CMOS imagers have a low voltage operation and low power consumption. Also, CMOS imagers are compatible with integrated on-chip electronics (control logic and timing, image processing, and signal conditioning such as A/D conversion). In addition, CMOS imagers allow random access to the image data. On-chip integration of electronics is particularly advantageous because of the potential to perform many signal conditioning functions in the digital domain (versus analog signal processing) as well as to achieve a reduction in system size and cost.
A number of methods have been introduced to extend the dynamic range of CMOS imagers. For instance, some approaches involve the acquisition of several images with different exposure times, with shorter times capturing brighter scenery and longer times capturing darker scenery. A high dynamic range image is then produced by combining the different images. Another method involves using multiple detectors to capture different copies of the same image, each detector being configured for a predetermined exposure time. Yet another method is based on comparator circuits that allow for saturation detection and reset of individual imaging pixels.
As appreciated from the above, current high dynamic range imaging techniques can either introduce unwanted artifacts or impose a large overhead cost, particularly as pixel sizes shrink and provide less signal output. Therefore, there is a need for new and efficient imaging sensors capable of high sensitivity and resolution over a wide dynamic range.
The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration preferred embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.